1. Field of the Invention
The present invention relates to a mirror, a method of manufacturing the same, an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
An exposure apparatus such as an EUV (Extreme Ultra Violet) exposure apparatus uses mirrors such as a multilayer mirror and a grazing-incidence total-reflection mirror. In the EUV range, the refractive index has a real part slightly smaller than one, so total reflection occurs upon grazing incidence in which EUV light strikes a reflection surface so as to graze it. Normally, upon grazing incidence in which light strikes a reflection surface at an angle below several degrees with respect to the reflection surface, a reflectance as high as several ten percent or more can be obtained, but the level of freedom of optical design is relatively low. Hence, a multilayer mirror formed by alternately stacking two substances having different optical constants (refractive indices) is useful as an EUV light mirror exhibiting a relatively high level of freedom of optical design. The use of a multilayer mirror makes it possible to obtain a desired reflectance even at an incident angle close to 90°, that is, upon approximately perpendicular incidence.
An EUV light multilayer mirror is formed by alternately stacking, for example, molybdenum and silicon on the surface of a glass substrate polished into an accurate surface shape. For example, the thickness of a molybdenum layer is 2 nm, the thickness of a silicon layer is 5 nm, and a composite film formed by these two layers is stacked by about 60 times. The sum of the thicknesses of layers formed by two substances will be referred to as a film period hereinafter. In the above-mentioned example, the film period is 2 nm+5 nm=7 nm.
When EUV light strikes a molybdenum/silicon multilayer mirror made of molybdenum and silicon, an EUV light component having a specific wavelength is reflected. Let θ be the incident angle, λ be the wavelength of the EUV light, and d be the film period. Then, only an EUV light component which has a narrow bandwidth with λ as its central wavelength so as to approximately satisfy the Bragg equation:2×d×cos θ=λ  (1)can be efficiently reflected. The bandwidth at this time is about 0.6 to 1 nm. The maximum reflectance is about 70%. FIG. 4 shows the wavelength dependence of the reflectance when EUV light strikes a multilayer mirror having a film period of 7.2 nm at an incident angle of 15°.
The molybdenum and silicon which form the multilayer film may react with each other at their interface. Thus, the wavelength exhibiting a peak reflectance may change or the reflectance may decrease. Japanese Patent Laid-Open No. 2007-155407 discloses an approach in which B4C or SiO2 is arranged between the molybdenum and the silicon as an intermediate layer.
The allowable shape error σ (rms value) is given by the Marechal equation:σ=λ/(28×√n)  (2)where n is the number of mirrors which constitute a projection optical system, and λ is the wavelength of the EUV light. Assuming, for example, six mirrors, the allowable shape error σ is 0.2 nm when the wavelength is 13.5 nm. The shape error can include a substrate shape error and film shape mirror. It is difficult to adjust the substrate shape and the film shape so that both of their errors fall below the allowable error. Japanese Patent Laid-Open No. 2005-19628 proposes a technique of correcting the shape of a mirror by thermal expansion using a heater attached to the mirror.
The technique described in Japanese Patent Laid-Open No. 2005-19628 requires constant control of the mirror temperature, which makes a temperature control mechanism indispensable, thus complicating the structure of a mirror unit.